A demand for a high frequency filter is rapidly enhanced because a wireless communication device such as a mobile phone is rapidly spread. In particular, an acoustic wave filter having a small size and having high precipitousness is demanded. The acoustic wave filter has an acoustic wave resonator. For example, in a high frequency band higher than 2 GHz band, a film bulk acoustic resonator is focused as a resonator having a small size and small loss. The film bulk acoustic resonator has a lower electrode, an upper electrode and a piezoelectric membrane between the lower electrode and the upper electrode. A region where the lower electrode and the upper electrode face with each other through the piezoelectric membrane is a resonance region contributing to a resonance. An acoustic wave propagating in a thickness direction of a resonance membrane contributes to resonance characteristics mainly. A spurious may occur when the acoustic wave propagating in the thickness direction propagates in a lateral direction.
Japanese Patent Application Publication No. 2006-109472 (hereinafter referred to as Document 1) discloses an art where the propagation of the acoustic wave in the lateral direction is restrained and the spurious is restrained by providing a thin membrane region that is a part of the upper electrode and is thinner than a center portion of the resonance region at an edge of the resonance region. Japanese Patent Application Publication No. 2007-6501 (hereinafter referred to as Document 2) and Proceeding of IEEE international ultrasonic symposium 2006, pp 456-459 (hereinafter referred to as Document 3) disclose an art where the propagation of the acoustic wave in the lateral direction is restrained more and the spurious is restrained more by providing a thick membrane region that is a part of the upper electrode and is thicker than a center portion of the resonance region outside of a thin membrane region.
However, with the method of Document 1, loss may be enlarged. With the methods of Document 2 and Document 3, a manufacturing cost may be enlarged when the upper electrode has three different thicknesses.